As water soaks into the dry soil, air is forced out of the surface layers which become saturated (waterlogged).
As water continues to enter the soil it moves steadily downwards, with a sharp boundary between the saturated zone and the dry, air-filled layers, as shown in Figure 19.2. So long as water continues to soak into the soil, this wetting front moves to greater depths and air is forced out of this zone.
When rainfall ceases the water in the larger soil pores continues to move downwards under the influence of gravity. Water is held in the soil in the form of water films around all the soil particles and aggregates. Forces in the surface of the water films, surface tension, hold water to the soil particles against the forces of gravity and the suction force of roots.
As the volume of water decreases, its surface area and hence its surface tension becomes proportionally greater until, in very thin films of water, it prevents the reduced volume of water from being removed by gravity. A useful comparison can be seen when your hands are lifted from a bowl of water. They drip until the forces in the surface of the thin film become equal to the forces of gravity acting on the remaining small volume of water over the hands.
Figure 19.2 Wetting front. As water is added to a dry soil it soaks into the soil with a clear line that can be seen between the unchanged (dry) soil and the saturated soil above
Gravitational water is the water that can be removed by the force of gravity.
Field capacity is the amount of water the soil can hold against the force of gravity.
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